Sodium carbonate forms a crystalline addition compound with hydrogen peroxide, corresponding to the formula 2 Na.sub.2 CO.sub.3.3 H.sub.2 O.sub.2 (sodium carbonate sesquiperoxide), and is commonly known as sodium percarbonate. The compound has many potential applications as a peroxygen source when dissolved in an aqueous medium.
Sodium percarbonate has a high active oxygen content (15.28% theoretical) and high water solubility. It is produced from low cost starting materials, and it is an environmentally safe chemical.
Sodium percarbonate has not achieved the commercial acceptance of sodium perborate, mainly because it is less stable than sodium perborate. Solid sodium percarbonate decomposes with a loss of active oxygen which is substantially greater than the decomposition of sodium perborate under the same conditions. This problem is particularly undesirable in detergent cartons sold at retail. The problem also is evident during processing and storage of detergent compositions.
Stabilizers such as magnesium sulfate are suitable for stabilizing sodium perborate, but provide only limited protection with sodium percarbonate. Various methods for stabilization of sodium percarbonate have been proposed.
U.S. Pat. No. 2,380,620 discloses that sodium silicate, magnesium sulphate or gum arabic are unsatisfactory stabilizers when incorporated in sodium percarbonate, but diphenylguanidine lessens the decomposition in the presence of the conventional stabilizers.
U.S. Pat. No. 3,951,838 discloses that prior attempts at chemical stabilization of sodium percarbonate, primarily by magnesium silicate, are generally ineffective in promoting long term stability, particularly in a humid atmosphere. The patent proposes coating of the particles with an aqueous silica sol and drying to accomplish stabilization.
U.S. Pat. No. 4,075,116 describes cocrystallizing of sodium percarbonate with other salts known to form perhydrates such as sodium sulfate, sodium pyrophosphate, sodium glucoheptonate, sodium perborate, and the like.
U.S. Pat. No. 4,171,280 discloses that a non-caking bleach composition may be formed containing up to 6% active oxygen by spraying only sufficient hydrogen peroxide onto sodium carbonate particles to convert a part of the sodium carbonate to sodium percarbonate. U.S. Pat. No. 4,260,408 teaches the addition of sodium phosphate to the composition as a stabilizer. Both patents demonstrate that an assay of less than 6% active oxygen (less than 40% sodium percarbonate) is necessary to obtain satisfactory stability.
U.S. Pat. No. 5,244,644 describes a process for producing alkali metal percarbonate with improved caking resistance which involves admixing percarbonate powder with a particulate fatty acid metal salt additive.
There is continuing research and development effort to produce sodium percarbonate in a form which exhibits long term stability under storage conditions, and when incorporated as a peroxygen ingredient in commercial products.
Accordingly, it is an object of this invention to provide a sodium percarbonate formulation which is stable under ambient temperature and moisture conditions.
It is a further object of this invention to provide a sodium percarbonate formulation which is stable when incorporated as an ingredient in a detergent or dentifrice type consumer product, and which releases active oxygen under product utilization conditions.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.